Vehicles may be configured with a manual transmission wherein a driver-operated clutch is engaged and disengaged by actuation of a foot pedal. Clutch adjustments may regulate torque transfer from the vehicle's engine to the transmission along a driveline. The vehicle operator may manually select a transmission gear via a gear selector and clutch adjustments be used in concert with the gear selection.
However, it is possible to compression lock an engine with a manual transmission. For example, during an engine start event, a cylinder is selected to be the first cylinder that receives both fuel and spark. Typically, the spark is scheduled to happen ˜10-15 degrees before cylinder top dead center (TDC). If the operator of the vehicle disengages the clutch at the moment of spark delivery, the torque produced by the firing cylinder may not be capable of moving the vehicle. As a result, the combustion pressure may force the engine to rotate backwards and the engine may stall. In addition, the combustion pressure may be trapped within the cylinder. Since the starter motor is not designed to overcome this large pressure, the engine may be locked until the combustion pressure leaks past the cylinder rings. This may take a significant amount of time, causing delays before an engine start can be attempted again. Delays in engine start time may cause operator frustration and degrade the vehicle's drive performance.
The issues described above may be addressed by a method for providing compression pressure relief from a locked engine. One example approach includes cranking an engine; and responsive to engine locking following the cranking, commanding a direct fuel injector of a cylinder open to relieve cylinder pressure into a high pressure fuel rail via the open direct fuel injector. In this way, a direct fuel injector may be used for compression pressure relief, allowing for a subsequent engine start to be attempted earlier.
As one example, during an engine start event, while the engine is being cranked, the output of a crankshaft position sensor may be monitored. If the sensor indicates that engine rotation is initially detected but then the rotation stops due to a change in state of a clutch pedal of a manual transmission, it may be inferred that the engine is compression locked. The compression pressure may then be relieved by commanding the direct injection fuel injector of the cylinder that just combusted (and is still before TDC) to open. The cylinder's compression pressure is then relieved via the injector into the high-pressure fuel rail. The air from the cylinder will dissolve into the fuel in the fuel rail and will be gradually expelled over the next several injection events. One or more fuel injection adjustments may be performed over those several injection events to account for the added air in the delivered fuel.
In this way, existing engine hardware may be advantageously used to relieve cylinder compression pressure. The technical effect of commanding open a cylinder direct injection fuel injector responsive to an engine being compression locked during cranking is that the pressure may be rapidly relieved, enabling an engine start to be reattempted soon afterwards. By selecting the cylinder whose direct injector is commanded open based on a spark timing of the cylinder relative to a timing of clutch application (following which the engine became compression locked), adjustments to a single cylinder can be used to unlock the engine. By reducing the compression pressure and expediting unlocking of the engine, operator frustration due to poor engine startability is reduced. Overall, performance of an engine configured with a manual transmission is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.